1. Field of the Invention
This invention relates generally to voltage reference supply units and, more particularly, to voltage reference supply units that are required to maintain an accurate voltage level over a wide range of temperatures.
2. Description of the Related Art
The stability of a voltage reference supply over a range of temperatures can provide the limiting factor for the accuracy of associated circuits. For example, in the military specification range (MILSPEC), the performance of an electronic device is specified over the temperature range of -55.degree. to +125.degree. C. In this temperature range, stability of the voltage level of the order of 500 parts per million can be typically obtained by providing compensating networks. More recently, some manufacturers provide voltage reference supply units with voltage level stability over this temperature range in the order of 300 parts per million using similar compensation techniques. These performance levels are typically achieved by using a Zener diode as a reference voltage source. The Zener diode is then coupled to temperature devices to compensate in a generally linear fashion for the temperature dependence of the voltage of the Zener diode.
However, the Zener diode can typically have non-linear components in the voltage level temperature dependence in this temperature range in addition to the linear component. It is the non-linear component of the output voltage level of the temperature dependence of the Zener diode which frequently provides the limit to the accuracy that can be achieved for the associated voltage reference supply.
A need has therefore been felt for a voltage reference supply that can operate in the temperature range of -55.degree. C. to +125.degree. C. with a variation in output voltage level of 50 parts per million or less over the entire temperature range.